A clutch unit of this type serves for the selective transmission of torque in a drive train of a motor vehicle. For this purpose, a friction clutch is provided which is typically configured as a multiple disk clutch and makes a continuously variable transmission of the torque possible. An associated actuator is configured in such a way that it exerts a force axially on the friction clutch. To this end, two ramp rings are used which have a plurality of ramps distributed over the circumference, which ramps are inclined with respect to a normal plane with respect to the rotational axis of the friction clutch and interact in pairs. Rolling bodies are preferably provided between the ramps of a pair. If at least one of the ramp rings is rotated (for example, by means of an electric motor or hydraulically), one of the ramp rings is moved axially as a result (the same ramp ring or the other ramp ring). By way of said axial movement, for example, a multiple disk assembly can be pressed together, in order to engage the clutch. U.S. Pat. No. 6,848,550 B2 has disclosed a clutch unit having an actuator which has ramp rings of this type.
One general problem is the development of heat in the case of a slipping friction clutch, for which reason the clutch has to be cooled. To this end, a cooling fluid, in particular oil, is usually used which is guided through the friction clutch. WO 2006/015394 A1 has already disclosed a transfer transmission of an all-wheel-drive motor vehicle, in which the clutch is supplied with lubricating oil. Here, the lubricating oil flows via an opening from an oil reservoir to the clutch.
DE 10 2007 057 984 A1 discloses an all-wheel-drive transfer transmission, in which, with the aid of a delivery device, lubricating oil can be conveyed from an oil sump into an oil reservoir in an upper region of the transmission. The oil flow from the oil reservoir to a friction clutch can be set by way of a metering device.
A disadvantage of cooling and lubricating of this type of a friction clutch is the drag torques which occur and the associated losses, in particular when the clutch is open. If a cooling fluid flows into the clutch in the open state, considerable losses occur on account of the viscosity. In particular in the case of multiple disk clutches, viscous friction is produced in the narrow gap between the clutch disks by way of the cooling fluid which flows in.
This problem has particular significance in what is known as a disconnect system, that is to say in a drive train with a torque transmission section which can be brought to a standstill and is arranged between said friction clutch and a separating clutch (positively locking clutch or likewise friction clutch). A torque transmission section which can be brought to a standstill can be provided, for example, in a motor vehicle having a permanently driven primary axle and a selectively driven secondary axle. In a motor vehicle of this type, the friction clutch can be arranged at an output of an all-wheel-drive transfer transmission and the separating clutch can be arranged in the region of an axle differential gear of the secondary axle (or vice versa). By way of opening of both clutches, the torque transmission section (for example, cardan shaft) which runs between them is brought to a standstill. This can prevent parts of the drive train which are not required in the two-wheel-drive mode (when only the primary axle is driven) also being driven, which would be at the expense of the fuel economy. A drive train of this type having a torque transmission section which can be brought to a standstill is known from DE 10 2009 005 378 A1.
However, an additional loss and therefore an additional heat input can also be produced by a cooling oil flow in a drive train and in transmissions, in which a standstill function of this type is not used, since the oil is conveyed continuously from an oil space into the clutch, is accelerated and moved in the process and is then swirled in the sump again, although no cooling would be necessary in the relevant driving situation.